1. Field of the Invention
The present invention relates generally to a seal construction for a fluid swivel joint, and more particularly, pertains to a seal construction for a large diameter, high pressure fluid and gas swivel joint, such as is commonly used on offshore loading terminals for oil and gas tankers.
2. Description of the Prior Art
The development of subsea petroleum and gas production systems has included a concept wherein a floating processing and storage vessel incorporates as a significant component thereof a high pressure multiple passage fluid swivel joint. the fluid swivel joint essentially includes therein a plurality of stationary fluid passages or pipes extending downwardly from the interior of the fluid swivel joint to carry gas, oil, air, water, or other fluids either up thereto or down therefrom. A plurality of couplings are mounted on the exterior of the fluid swivel joint and are rotatable relative thereto, with each rotatable coupling communicating through the fluid swivel joint with one of the stationary fluid passages or pipes. Griebe U.S. Pat. No. 2,894,268 and Briatianu U.S. Pat. No. 3,590,407 disclose float supported sea terminals utilizing a concept very similar to that described above.
A problem associated with such a fluid swivel joint is in the provision of seals which will withstand the relatively high pressure differentials, often up to 6000 psi thereacross, while also providing for the relative rotational movements associated with the swivel joints. The seals which have been utilized in many of these fluid swivel joints have been V-type lip seals, which often resulted in very large frictional forces in the swivel joints when they are designed to accommodate the high pressure differentials.
Additionally, the high pressure differentials have also resulted in extrusion of the relatively pliant sealing members into the gaps which they are designed to seal. Prior art swivel joints have often been constructed with an outer ring which rotates about an inner ring with radial seals therebetween. High internal pressures often result in deflections in the rings which increase the radial clearance gap. Common seal materials cannot effectively bridge this gap while maintaining the pressure integrity of the seal. Therefore, common seal materials have been reinforced with various elements to increase the materials' physical strength in order to bridge the radial clearance gaps. Unfortunately, the reinforcing elements tend to render the seal material compounds less resilient and more abrasive. Consequently, a seal formed of these compounds must have a high contact force in order to effect a seal because the less resilient materials have increased resistance to the filling into microgrooves in the mating seal surface so as to seal against any fluid passage. The higher contact stress and the more abrasive nature of the reinforcing elements of the material compounds tend to increase wear of both the seal and the seal mating surface, frequently leading to loss of pressure integrity of the seal. In order to solve the problems of extrusion of the seal materials into the gap, the prior art has also utilized anti-extrusion rings of hard plastic or metal to support sealing elements and bridge the clearance gap between the rings of a fluid swivel joint. Unfortunately, these prior art designs have not proven to be totally satisfactory.
In order to ameliorate the problems encountered in prior art seal constructions of this type, U.S. patent application Ser. No. 454,826; filed on Dec. 30, 1982; and assigned to the common assignee of this application, and now U.S. Pat. No. 4,555,118, discloses improved sealing arrangements for a fluid swivel joint such as is commonly used in offshore loading terminals for tankers. Among various disclosed embodiments is a sealing arrangement which is designed with a bias-loaded anti-extrusion ring or seal to minimize the seal extrusion gap therein.
Also disclosed is a sealing arrangement for a fluid swivel joint wherein the components of the seal are relatively free-floating in order to compensate for eccentricity or other variations or distortions in the components of the fluid swivel joint.
Pursuant to the copending U.S. patent application Ser. No. 454,826 a sealed fluid joint has first and second adjacent joint rings which are rotatable relative to each other about a common central longitudinal axis. The joint rings have a small annular, ring-shaped clearance gap provided therebetween so as to allow for relative rotational movement, and one of the rings defines an annular seal housing groove adjacent to the clearance gap. An annular seal is positioned in the seal housing groove, and its components include an annular sealing member and an adjoining anti-extrusion ring to prevent the pliant sealing member from being extruded into the clearance gap by the relatively high fluid pressure differential existing across the seal. The anti-extrusion ring or the sealing member is biased against an adjacent sealing surface to provide an effective high pressure fluid seal therebetween and also to minimize the extrusion gap existing within the seal. In several embodiments, the swivel joint defines a radial seal configuration having a cylindrically shaped clearance gap extending symmetrically around the central longitudinal axis. In other alternative constructions, the swivel joint defines a face seal configuration having a radially flat-shaped clearance gap positioned symmetrically around the central longitudinal axis.
Moreover, the biasing action is achieved by a spring which is positioned in the seal housing groove so as to bias the annular sealing member against a sealing surface on the second joint ring. In one embodiment, a coil spring is mounted in compression, extending between a wall of the seal housing groove and the annular seal. In another embodiment, a spring extends circumferentially around the annular seal in the annular seal housing groove. In several other embodiments, the spring biases the anti-extrusion ring against the annular sealing member to cause it to bear against a sealing surface on the second joint ring and to also minimize the seal extrusion gap within the seal.
In various of the seal designs, the seal utilizes the pressure differential existing across the seal to pressure bias the annular sealing member against a sealing surface and also to minimize the seal extrusion gap. Other designs allow for the pressure differential to bias the anti-extrusion ring against the annular sealing member to cause it to bear against a sealing surface on the second joint ring and also to minimize the anti-extrusion gap. In one disclosed embodiment, the pressure differential is utilized to directly bias the annular sealing member against a sealing surface.
In several of the seal designs, the annular sealing member is formed with a V-shaped concave slot therein extending along its annular length to take further advantage of the pressure differential, while in other seal designs, the anti-extrusion ring is constructed with an L-shaped cross-section, and the annular sealing member is positioned between the two legs of the L. In one design, the V-shaped concave slot has its V-shaped opening facing in the direction of the annular clearance gap, while in another the V-shaped opening faces orthogonally away from the annular clearance gap.
Moreover, several multi-element seals are disclosed in embodiments designed to eliminate problems associated with prior art seals of this kind, such as extrusion of the sealing element and unnecessary wear of both the seal mating surfaces. Several of these seal designs include a secondary sealing member positioned intermediate the pliant sealing member and the anti-extrusion ring. In these embodiments, O-rings are positioned between the pliant sealing member and the secondary sealing member and also between the secondary sealing member and the anti-extrusion ring.
The anti-extrusion rings may be constructed of separate pieces or, alternatively, may be an integral component of the seal.
As long as the inner and outer rings of the swivel joint are in a concentric condition relative to each other, the anti-extrusion rings or devices will perform their sealing function in a satisfactory manner. However, operating conditions are encountered at times wherein the inner and outer rings of the swivel joint become eccentric relative to each other, so as to cause any anti-extrusion ring which is composed of plastic material to be either fractured or deformed to the point of becoming ineffective. Under similar conditions, when the anti-extrusion rings are constructed of metal, they can readily damage the seal surfaces of the inner and outer swivel joint rings as a consequence of their relative eccentricity. Moreover, the eccentricity which may be encountered between the inner and outer rings will cause the non-uniform radial loading of the seal so as to adversely affect the seal pressure integrity, and to produce uneven seal wear tending to result in premature seal failure.